1. Field of the Invention
The present invention relates to a field effect transistor which uses a gallium nitride-based compound semiconductor, and a method of manufacturing the same.
2. Description of the Related Art
A gallium nitride-based compound semiconductor has a wider forbidden band, so that a field effect transistor (FET) using such a semiconductor can be operated at higher frequencies and at higher voltages. In view of the FET's applicability as a high-output power semiconductor device, there have been proposed a metal semiconductor FET (MESFET), a high electron, mobility transistor (HEMT), and the like. A HEMT is a high-speed semiconductor device that is popularly used for operation at higher frequencies, and specifically, devices using a GaAs/AlGaAs heterojunction have been used for practical applications. Excellent microwave/millimeter wave characteristics of HEMTs have met widespread use for a low-noise, high-speed field effect transistor, e.g., for a satellite broadcasting receiver (refer to Japanese Patent Application Laid-Open No. 2003-297856, No. 2000-174285 and No. 2004-319552, for example).
In recent years, however, as the next-generation high-speed FET, more interests have been focused on a HEMT using a GaN-based compound (hereinafter referred to as a GaN-based HEMT) in place of a GaAs-based compound. Extensive researches have been repeated, because a GaN-based compound has a wide band gap and high saturated electron velocity which is estimated from electron effective mass, also because the GaN-based compound ensures a possibility that a high frequency device is realized for operation at higher power outputs, at higher breakdown voltages, and at a higher temperature range. FIG. 7 illustrates an example of HEMT structure using a GaN-based compound. The illustrated GaN-based HEMT 700, in a sequential lamination from the bottom on an insulating sapphire substrate 71, contains an AlN buffer layer 72, an undoped GaN layer 73 as an electron transit layer, and an n-type AlGaN layer 74 as a carrier supply layer. Also, formed on the upper surface of n-type AlGaN layer 74 are a source electrode 75, a gate electrode 76, and a drain electrode 77. In a HEMT with this structure, the n-type AlGaN layer 74 serving as a carrier supply layer supplies electrons to the undoped GaN layer 73 serving as an electron transit layer, and thus such supplied electrons transit, with high mobility, through the region 73a which serves as a channel contacting the n-type AlGaN layer 74 at the uppermost portion of the GaN layer 73.
In order to achieve a high power output for the above-described HEMT, even a higher breakdown voltage must be so designed that a high voltage can be applied. In operation, however, when a higher voltage is applied between the source electrode 75 and the drain electrode 77, a portion of the device will be subjected to a large concentration of electric field, resulting in a device breakdown. In particular, a short circuit becomes problematic, which results from a dielectric breakdown between gate and drain electrodes. In a laterally structured device as illustrated in FIG. 7, where the source electrode 75, the gate electrode 76, and the drain electrode 77 are formed on the same plane, it would be difficult to maintain insulation, above several hundred volts, between these electrodes. Even when a certain extent of insulation can be maintained between the source and drain electrodes, application of a high voltage of as much as several thousand volts will cause a creeping discharge and the like, and a dielectric breakdown will problematically occur due to air existing between the source and drain electrodes or due to a failure of breakdown voltage in the insulation layer which covers these electrodes. In addition, with the above-described arrangement which is so structured that electric field easily get concentrated directly beneath the drain electrode, a dielectric breakdown is very likely to occur because a high voltage will be applied by a concentration of electric field while in operation at a high voltage. Such a dielectric breakdown has thus been hampering higher power outputs for HEMT devices.
The present invention has been made in order to solve the above-described problems. A main object of this invention is to provide a field effect transistor which realizes higher breakdown voltages to achieve higher power outputs, and to provide a method of manufacturing the same.
To attain the above object, a field effect transistor (FET) according to a first aspect of the present invention includes a source electrode, a gate electrode, a drain electrode and a semiconductor structure which contains a carrier transit layer. The semiconductor structure is made of a Group III-V compound semiconductor layer; disposed on or above the semiconductor structure are the gate electrode and the source electrode; and the drain electrode is disposed on opposing side of the semiconductor structure where the gate electrode is disposed. This arrangement enables a drain's breakdown voltage, especially breakdown voltage for the inter-electrode of drain-source or drain-gate to be increased in the FET, because the gate and source electrodes and the drain electrode are respectively disposed, in a spatial separation of each other, on different planes instead of the same plane of the semiconductor structure. With this arrangement, the FET also can be of a vertical FET semiconductor structure having a channel between the drain and source electrodes.
Also, in the field effect transistor according to a second aspect of the present invention, the semiconductor structure contains a first semiconductor layer and a second semiconductor layer which has a band gap energy greater than that of the first semiconductor layer; the gate electrode and the source electrode are disposed respectively on the first semiconductor layer side of the semiconductor structure; and the drain electrode is disposed on the second semiconductor layer side of the semiconductor structure. This arrangement enables a carrier to transit at higher mobility in a channel.
Furthermore, the field effect transistor (FET) according to a third aspect of the present invention includes a source electrode, a gate electrode, a drain electrode and a semiconductor structure which contains a first semiconductor layer and a second semiconductor layer having a band gap energy greater than that of the first semiconductor layer. The first and second semiconductor layers are made of a gallium nitride-based compound semiconductor layer; disposed on the first semiconductor layer side of the semiconductor structure are the gate electrode and the source electrode; disposed on the second semiconductor layer side is the drain electrode and the drain electrode is disposed on opposing side of the semiconductor structure where the source electrode is disposed. This arrangement enables an FET's breakdown voltage to be increased, because the source and drain electrodes are respectively disposed, such as the invention according to the above-described first and second aspect, on the opposing planes. With this arrangement, the source and gate electrodes are disposed on the first semiconductor layer, and the drain electrode is disposed on the second semiconductor layer, so that a concentration of electric field can be mitigated between the drain and gate electrodes.
Furthermore, in the field effect transistor according to a fourth aspect of the present invention, the second semiconductor layer has an AlGaN layer.
Furthermore, in the field effect transistor according to a fifth aspect of the present invention, the second semiconductor layer has an undoped AlGaN layer and a Si-doped AlGaN layer. With this construction, a carrier scattering can be reduced at the interface of the first and second semiconductor layers.
Furthermore, in the field effect transistor according to a sixth aspect of the present invention, the carrier transit layer contained in the semiconductor structure has an end surface exposed on a side where the source electrode is disposed, and the source electrode is disposed to make an ohmic contact with the end surface. With this arrangement, even when the second semiconductor layer is an i-type GaN layer and the like, an electric current can be extracted to realize a field effect transistor with a high breakdown voltage.
Furthermore, in the field effect transistor according to a seventh aspect of the present invention, there is an intermediate layer disposed between the second semiconductor layer and the drain electrode. With this arrangement, the drain electrode and the second semiconductor layer can be spaced apart, so that a concentration of electric field can be mitigated by an increased distance between the drain and gate electrodes, resulting in an even higher breakdown voltage of the field effect transistor.
Furthermore, in the field effect transistor according to a eighth aspect of the present invention, the intermediate layer is made of GaN. With this construction, it is possible to prevent an extreme concentration of electric field into the gate electrode terminal, reducing an effect of surface level, so that the field effect transistor can be stably operated.
Furthermore, in the field effect transistor according to a ninth aspect of the present invention, the first semiconductor layer is a carrier transit layer, and the second semiconductor layer is a carrier supply layer.
Furthermore, in the field effect transistor according to an tenth aspect of the present invention, the field effect transistor is a high electron mobility transistor (HEMT).
Furthermore, in the field effect transistor according to a eleventh aspect of the present invention, the gate electrode is annularly disposed so as to surround an area where the source electrode is disposed, while the drain electrode is, further, annularly disposed so as to surround an area on opposing side of the semiconductor structure where the gate electrode is disposed. With this arrangement, the gate electrode is disposed between the drain and source electrodes, and the gate and drain electrodes are annularly disposed to prevent a concentration of electric current, resulting in realization of a high breakdown voltage.
Furthermore, in the field effect transistor according to a twelfth aspect of the present invention, the gate electrode being annularly disposed so as to surround an area on opposing side of the semiconductor structure where the drain electrode is disposed, while the source electrode is, further, annularly disposed so as to surround an area where the gate electrode is disposed. With this arrangement, the gate electrode is disposed between the drain and source electrodes, and the gate and drain electrodes are annularly disposed to prevent a concentration of electric current, resulting in realization of a high breakdown voltage.
Furthermore, a method of manufacturing a field effect transistor according to a thirteenth aspect of the present invention is in fabrication of a field effect transistor provided with a carrier transit layer and a carrier supply layer, the carrier supply layer being formed on or above the carrier transit layer and having a band gap energy greater than that of the carrier transit layer. The method includes a step of sequentially laminating at least the carrier transit layer and the carrier supply layer upon a substrate so that a semiconductor structure is formed; a step of forming a drain electrode on one side of the carrier supply and transit layers of the semiconductor structure; a step of forming a gate electrode and a source electrode on opposing side of the semiconductor structure where the drain electrode is formed; and a step of securing a side of the semiconductor structure where the drain electrode is formed or where the gate and source electrodes are formed to a support substrate and then removing the previously stated substrate. With this method, the gate and source electrodes and the drain electrode are respectively disposed on different surfaces instead of the same surface of the semiconductor structure, so that a field effect transistor is obtained which has high breakdown voltages between the gate and drain electrodes and between the source and drain electrodes.
Furthermore, in a method of manufacturing a field effect transistor according to a fourteenth aspect of the present invention, the drain electrode is disposed between the semiconductor structure and the support substrate. With this arrangement, a highly reliable insulation structure can be obtained because a portion between the source and gate electrodes is disposed on a side of the semiconductor structure, opposite the support substrate.
Furthermore, in a method of manufacturing a field effect transistor according to a fifteenth aspect of the present invention, the support substrate is a conductive substrate and secures to the semiconductor structure through a conductive material; and the drain electrode is electrically connected to the support substrate and also has an external connection on opposing side of the support substrate where the semiconductor structure is disposed. With this arrangement, a vertical FET structure wherein the support substrate is an extraction electrode of the drain electrode can be obtained.
Furthermore, in a method of manufacturing a field effect transistor according to a sixteenth aspect of the present invention, in the step of forming the semiconductor structure, an intermediate layer is laminated on or above the carrier supply layer, and the drain electrode is disposed on or above the intermediate layer, with the intermediate layer being partially disposed on or above the carrier supply layer.
According to a field effect transistor and a method of manufacturing the same in accordance with the present invention, high insulation properties can be attained by an arrangement where the gate electrode, the source electrode, and the drain electrode are spaced apart of each other, so that an increased breakdown voltage between these electrodes makes it possible to obtain a highly reliable field effect transistor which can be operated at a high power output. An increased breakdown voltage for the drain electrode, in particular, can improve a breakdown voltage of the field effect transistor.